1. Field of the Invention
This invention relates generally to direct conversion receivers, and more specifically, to direct conversion receivers employing a mixer configured to reduce the effects of leakage between the radio frequency and local oscillator input ports thereof.
2. Background
A radio receiver of the type currently employed in mobile wireless phones is illustrated in FIG. 1. As shown, the receiver comprises a first mixer 1, having a radio frequency (RF) input port 5 coupled to an antenna 11, and a local oscillator (LO) input port 8. The mixer has an output port 12 coupled, through signal line 6, to the input of a band-pass filter (BPF) 3. The BPF 3 has an output coupled to the intermediate frequency (IF) input port 10 of a second mixer 2. The second mixer 2 also has a local oscillator (LO) input port 9. The output port 14 of mixer 2 is coupled, through signal line 15, to the input of low-pass filter (LPF) 4. The output of LPF 4 is coupled to signal line 7.
First mixer 1 is configured to multiply the signals received at the RF and LO input ports thereof, and provide the multiplied signal at the output port thereof. The frequency of the signal received at the RF input port is fRF, and the frequency of the signal received at the LO input port is fLO1. The signal received at the RF input port is derived from a signal received over antenna 11. Typically, this signal represents a digitized audio signal which has been modulated onto an RF carrier signal. In the following discussion, this digitized audio signal will be referred to as the baseband signal, but it should be appreciated that, in practice, the baseband signal can be a desired signal other than a digitized audio signal, including a data signal.
The signal provided at the output port 12 will have first order components at frequencies fRFxe2x88x92fLO1 and fRF+fLO1. The frequency fRFxe2x88x92fLO1 is an intermediate frequency which will be referred to as fIF. In one implementation, fRF is 900 MHz, fLO1 is 450 MHz, and fIF is 450 MHz. In this implementation, the first order components of the output signal will be at 1350 MHz and 450 MHz.
BPF 3 has a passband centered at fIF, and is configured to allow passage of the IF component of the output signal, and to prevent passage of the other first order component, that is, the component at the frequency fRF+fLO1. BPF 3 also rejects any unwanted signals outside the desired band around fRFxe2x88x92fLO1. This IF component is then provided as an input to input port 10 of mixer 2.
The signal provided as an input to the LO port of mixer 2 has a frequency fLO2. This frequency is selected so that it is the same as the frequency fIF of the signal provided at the input port 10. Mixer 2 multiplies these two signals, and provides the multiplied signal on output port 14. The output signal will have two first order components, one at the baseband frequency, fBB, and the other at twice the intermediate frequency fIF.
The output signal from mixer 2 is provided as an input to LPF 4. LPF 4 is configured to allow passage of the baseband component of the signal output from mixer 2, and prevent passage of the high frequency component, that is, the frequency at twice fIF, of the output of mixer 2. The baseband component is thus provided as an output of the receiver line 7.
In operation, the receiver of FIG. 1 functions as follows. A signal is received over antenna 11 representing a baseband signal modulated onto an RF carrier signal. The signal is passed through mixer 1, which produces at its output port a signal having a first order component representative of the baseband signal, but at an intermediate frequency rather than baseband frequencies, and also a second first order component. The signal is passed through BPF 2 to isolate the intermediate frequency component from the other first order component. This intermediate frequency component is then passed through mixer 2, which produces at its output port a signal having a baseband component and an intermediate frequency component. The signal is then passed through LPF 4 to isolate the baseband component from the higher frequency component. LPF 4 thus produces at its output a signal representative of the baseband signal.
As is apparent from the foregoing description, the operation of the receiver of FIG. 1 proceeds in two basic steps. In the first step, the baseband portion of the incoming RF signal is down converted to an intermediate frequency. In the second step, the baseband portion at the intermediate frequency is down converted to the baseband frequency. Each of these steps is performed on and through distinct elements, the first step, through mixer 1 and BPF 3, and the second step, through mixer 2 and LPF 4.
Because of the cost and complexity of downconverting the baseband portion in multiple steps in the receiver of FIG. 1, and the cost of the elements needed to perform these multiple steps, the receiver of FIG. 1 is not ideal.
Accordingly, an object of the subject invention is a receiver which overcomes the disadvantages of the prior art. Additional objects and advantages will be apparent to those of skill in the art who practice the invention or will be set forth in the disclosure which follows.
3. Related Application
This application is relation to a co-pending patent application entitled xe2x80x9cPREPROCESSOR AND RELATED FREQUENCY TRANSLATOR,xe2x80x9d Lyon and Lyon Dkt. No. 240/111, Serial No. To Be Assigned, which is owned in common by the assignee hereof, and is being filed on even date herewith. This application is hereby fully incorporated by reference herein as though set forth in full.